NOTE: The most common method of engine mounting is overhead suspension.
However, at some engine test facilities, the engine is mounted on a pedestal supported by the test cell floor. The overhead mount more closely simulates the mounting in many aircraft, and more easily accommodates the cleaning of the engine test section and the accessibility of bottom-mounted engine accessories. The engine mount should be designed to prevent transverse motion, fishtailing, or any type of lateral instability. With turbofans, poor lateral stability due to mount flexibility can result in severe engine oscillations during testing. Thrust mount designs should also ensure that engine axial alignment is maintained during testing.
See Figure 2-4 for a schematic representation of a typical overhead mount installation.
FIGURE 2-4. TYPICAL THRUST MOUNT DESIGN
d. Test Cell Inlet System. The test cell inlet system conditions incoming air to reduce the effects of wind speed, direction, and extreme temperatures. This system consists of flow straighteners, heaters, screens, and noise suppressers. These components tend to create a pressure loss or Pcd. The following test cell inlet design features may have an influence on engine performance:
/g183/g32 Area
/g183/g32 Blockage
/g183/g32 Turning angle
/g183/g32 Axial distance between engine inlet and exit of the cell inlet
/g183/g32 The angle of inlet turning vanes
Par 202 Page 9
e. Test Cell Exhaust System. The test cell exhaust system augmentor removes engine exhaust gases from the engine test section, induces the flow of secondary air for cooling, and provides some noise abatement. The mix of exhaust gases and the cooling secondary airflow that goes through the augmentor is then directed through an exhaust stack prior to exiting the facility. The following test cell exhaust design features may have an influence on engine performance:
/g183/g32 Augmentor configuration (i.e., convergent or divergent)
/g183/g32 Augmentor tube length and diameter
/g183/g32 Exhaust inlet tube diameter
/g183/g32 Axial distance between engine exhaust and augmentor inlet
/g183/g32 Area ratio of the engine exhaust to the augmentor area Page 10 Par 202
CHAPTER 3. MEASUREMENT SYSTEMS, CALIBRATION, AND CORRELATION
300. MEASUREMENT SYSTEMS. The following paragraphs provide a general description of typical measurement systems. Instrumentation quantity, position, range, and accuracy are usually specified in an OEM’s document, such as a facility planning manual or overhaul manual.
NOTE: End-to-end checks of each of the systems addressed in this section are extremely important to assure the validity and accuracy of the data being collected.
These checks involve the comparison of data from a particular readout with its associated primary or secondary standard or with a known source. During these system checks, many physical, electrical, or flow discrepancies can be detected and corrected before correlation testing begins.
a. Pressure Measurement Systems. Pressure measurement systems usually use simple pressure gauges, individual dedicated pressure transducers, or a scanning-valve system. Scanning-valve systems are typically a single pressure transducer that alternately measures many pressure lines of similar pressure range.
NOTE: High humidity or rain drawn into the test cell can result in water getting into the pressure lines and producing erroneous pressure measurements for both types of pressure management systems. (See section 305a(2) of this AC.)
b. Temperature Measurement Systems. Temperature measurement systems should be capable of independently evaluating “low and medium” and “high” temperatures. Commonly, low and medium temperature measurement systems utilize E- or J-type thermocouples to measure engine cold section and facility temperatures. High temperature systems typically use K-type thermocouples to measure exhaust gas temperatures. The parameters to be measured will depend on the OEM’s requirements for testing.
Thermocouple junction locations should be minimized within the test cell environment to reduce system noise and error.
c. Engine Speed Measurement System. The engine speed measurement system must be capable of measuring all engine speeds. A typical system may consist of a digital electronic frequency counter device and a readout device, or an analog type with a graduated display dial readout. Both systems provide engine speeds in revolutions per minute or percentages.
d. Fuel Flow Measurement Systems. A typical fuel flow measurement system may consist of one or more turbine flow meters (or comparable flow measuring device(s)), each connected to a digital frequency counter and display gauge. This total measurement system, from the flow meter through the frequency counter to the display gauge, must be checked end-to-end for the entire operating range of the system. For example, a defective frequency counter could provide an acceptable error in the low flow range, while generating a divergently increasing unacceptable error in the high range.
NOTE: Fuel flow measurement becomes increasingly important when it is used in turbine gas temperature calculations. This calculation is sometimes required by OEMs for acceptance testing.
e. Vibration Measurement System. Vibration is not typically evaluated during test cell correlation testing because it does not normally affect engine performance. However, turbine engines cannot be properly evaluated during in-service acceptance testing, as required by the overhaul manual, without employing a system to provide vibration measurement. System components can include vibration pickups, amplifiers, tracking filters, spectrum analyzers, band pass filters, and audio oscillators. Engine Par 300 Page 11
pickup locations can include the fan case, intermediate case, diffuser case, and turbine exhaust case. Hot section vibration pickups may require insulation to prevent high-temperature damage.
f. Trim Balance System. Although trim balancing of the low speed compressor is not required for test cell correlation, performing such balancing before correlation testing is acceptable. In addition, the OEM may specify a trim balance during in-service engine acceptance testing to determine imbalance moment values and, thus, the location and weights of balance counterweights. Generally, trim balancing is accomplished with the aid of a trim balance analyzer. However, procedures are also available for trim balancing with trial balance weights.
g. Thrust Measurement Systems. Thrust measurement is typically accomplished by either an electronic strain gauge or a hydraulic measurement system. The required system accuracy should be specified by the OEM.
301. INSTRUMENT CALIBRATION.
a. Engine Test Facility. The test cell can be considered a component of an engine’s test equipment.
As such, it requires instrumentation calibration and an end-to-end check of the measurement systems.
Instrument calibration plus system checks ensure acceptable measurement accuracy within engine acceptance testing limits specified in the OEM’s overhaul manual. The OEM’s overhaul manual and other documents, such as a facility planning manual, reflect the procedures and test equipment that are required to test engines and the parameters to be measured. Test cell instrumentation must be calibrated and traceable to an accepted, known national standard.
b. Standards. In the United States, the NIST represents the United States at the International Conference on Weights and Measurements and has the primary responsibility for maintaining the national and international standard units of length, mass, time, temperature, and electrical quantities. Other nations have comparable standards bodies. An acceptable secondary or transfer standard has been generally regarded as any standard that is at least four times more accurate than the accuracy requirement for the instrument being calibrated and is traceable to a national standard, such as those maintained by NIST.
c. Calibration of Instruments and a Check of Associated Systems. Calibration is used to check an instrument’s measurements against an accepted known standard and to eliminate unacceptable measurement errors. Instruments should be calibrated over the expected range of operation. Periodic calibration of individual instruments is necessary and can be performed in place or in the instrumentation shop.
d. Traceability. Traceability of each instrument calibration is important to show where all possible error contributions occur between the instrument and the standard. Traceability should be well documented. The calibration hierarchy, with inherent accuracies at each point, must be documented.
Traceability back to a specific standard does not reduce the uncertainty of a measurement; instead, it provides clear documentation of the process of its determination.
302. THRUST CALIBRATION. To calibrate the load cell, a common procedure is to apply known centerline forces using a centerline pull rig. This calibration procedure results in the development of a calibration curve of the known force and the measured load cell reaction. This curve is used during the correlation to obtain a true force from the load cell measurement. Another method is the use of a calibration device built into the thrust stand.
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303. FACTORS INFLUENCING CORRELATION. The following factors may influence the correlation of an engine test cell:
/g183/g32 Configuration of the test cell (particularly the test cell inlet, engine test section, and augmentor tube and exhaust stack configurations)
/g183/g32 Engine position in the test cell
/g183/g32 Ambient conditions and surrounding area configurations
/g183/g32 Instrumentation (including the calibration, location, measurement accuracy, and quantity)
/g183/g32 Test cell thrust measurement system
/g183/g32 Test procedures
/g183/g32 Data acquisition system
/g183/g32 Fuel properties
/g183/g32 Cell airflow
NOTE: In addition, engine test hardware, such as cowling inlets, bellmouths, QEC packages, cowling thrust reversers and noise suppressers, and exhaust nozzles installed on the engine during testing may greatly influence the data. These components should be serialized for tracking control.
304. PERFORMANCE MEASUREMENT.
a. The primary function of the engine test facility is to obtain a proper performance evaluation of an engine. The test facility must provide a stable test environment conducive to a smooth, surge free operation of the specific engine model. The environment of test facilities always has an influence on the test data. This is particularly true of indoor test cells. In addition, the engine test hardware configuration influences the test data. The combined effects of the test facility and engine test configuration on performance parameters are generally the reason for differences in an engine’s performance between test cells. The test cell correlation provides a means to:
/g183/g32 Quantify these differences with respect to a standard
/g183/g32 Understand these differences
/g183/g32 Eliminate or reduce these differences, whenever possible
/g183/g32 Establish appropriate cell correlation factors
b. Performance Parameters. During a test cell correlation, a variety of engine performance parameters should be examined, including those parameters specified in the overhaul manual for engine acceptance testing and any values or factors used to correct primary parameters. Parameters may include, but are not limited to, the following:
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/g183/g32 Engine fuel flow
/g183/g32 Engine thrust
/g183/g32 Engine rotor speeds
/g183/g32 Engine airflows
/g183/g32 Engine pressures and temperatures
/g183/g32 Engine inlet conditions
/g183/g32 Test cell temperatures and pressures
/g183/g32 Test cell inlet airflow
/g183/g32 Engine control systems and variable geometry
/g183/g32 Barometric pressure
/g183/g32 Ambient humidity
NOTE: Sufficient data must be obtained to establish the performance of the engine.
Secondary parameters used in the derivation of any of the above items must also be measured and recorded. As an example, a fuel sample should be taken during the test and accurately analyzed for a lower heating value (LHV) and specific gravity value. In addition, redundant instrumentation can be useful for data verification and troubleshooting purposes.
305. FACTORS AFFECTING PERFORMANCE. To compare performance parameters between various test facilities or conditions within the same test facility, correcting or normalizing the performance parameters to common reference conditions is necessary. These reference conditions fall into three groups: Ambient (humidity, temperature, and pressure); fuel properties (relative density, viscosity, and LHV); and aerodynamic (ram pressure ratio, cell depression, and cell bypass airflow interaction).
a. Humidity. While it has been recognized for a long time that high humidity levels will affect the performance of gas turbine engines, no consensus exists on how to handle the associated effects or problems. Water vapor contained in the air may have several influences on the engine and its performance. Although the consequences are complex, they fall into two major categories: engine inlet condensation and changes in engine gas properties. While the relative humidity is related to the extent of engine inlet condensation, the absolute or specific humidity affects the gas properties of the engine cycle and, hence, the performance elements.
(1) Actual condensation in an engine inlet depends on a series of factors, such as relative humidity, air temperature, air pressure, inlet Mach number, and dwell time. For given humidity conditions, the probability for condensation is higher in long inlet ducts and lower in bellmouth intakes.
Bellmouths should be sized per the OEM’s specifications. For example, an undersized bellmouth could result in a high local inlet Mach number that might cause inlet condensation at lower relative humidities.
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(2) Humidity corrections for most performance parameters have been found to be small.
However, humidity can have an impact on performance and should be considered when accurate performance measurements are required. To minimize this effect, some test facilities and OEMs choose to impose limits on test cell humidity during correlation testing for particular engine models. A relative humidity restriction of 75 percent is not uncommon for correlation testing.
b. Engine Inlet Temperature and Pressure. All gas turbine engines are affected by the ambient conditions in which they operate (primarily air temperatures and pressures). Also, engine operation and correlation can be affected by an engine inlet temperature distortion or gradient due to exhaust gas recirculation or other sources of heat. This gradient should be minimized. Unstable and low pressures in the test chamber or engine inlet also affect engine operation and correlation. Distorted/non-uniform cell airflow can cause vortices and asymmetric pressure distribution in the engine inlet. These conditions can also be caused by exhaust gas recirculation, poorly designed test cell inlet and test chamber sections, too many items protruding into the inlet airflow stream, and marginal chamber secondary airflow. In most cases, unacceptable temperature or pressure conditions are obvious because of their effect on engine performance. Their cause should be corrected to the extent possible before correlation testing.
(1) It is usually not possible to control the engine inlet air temperature and pressure to sea level, static, standard day values. Therefore, to compare one engine run to another, the measured engine performance parameters must be adjusted to the values they would have with standard day inlet air. This process is called normalizing or making standard day corrections to the measured data.
(2) The method for correcting performance parameter data often varies between different engine types and models. The procedures to correct parameters for a particular engine model can be found in a specification, Technical Standard Order, or the OEM’s engine overhaul manual.
(3) Certain primary operating variables of a gas turbine engine are normalized or corrected as functions of total temperature and total pressure at the engine inlet. The basic normalizing parameters are as follows:
(a) θ (theta) = (observed inlet total absolute temperature) divided by (absolute temperature of International Standards Organization [ISO] sea level, static, standard day reference atmosphere).
(b) δ (delta) = (observed inlet total absolute pressure) divided by (absolute pressure of ISO sea level, static, standard day reference atmosphere).
NOTE: These ratios require the use of consistent units and absolute values (e.g., temperatures in degrees Kelvin or degrees Rankine and pressures in psia, in-HgA, or kPa).
NOTE: In some cases, measured data from certain gas turbine engines are corrected to conditions other than ISO standard day values. In these cases, refer to the applicable engine model specification, Technical Standard Order, or engine overhaul manual.
(4) The correction or normalizing of the major engine performance parameters requires the use of θ and δ, as follows:
(a) Rotor speed, N, is normalized when divided by θx (i.e., N/θx) where X is dependent on the engine type and defined by the manufacturer (commonly, 0.5 is used).
Par 305 Page 15
(b) Thrust, F, is normalized when divided by δ (i.e., F/δ).
(c) Airflow rate, Wa, is normalized when multiplied by √(θx/δ) (i.e., Wa√(θx/δ)).
(d) Fuel flow rate, Wf, is normalized when divided by δθy (i.e., Wf/δθy) where Y is dependent on the engine type and defined by the manufacturer (typical values of Y range from 0.5 to 0.7).
(e) Engine cycle absolute temperatures (such as T3, T4, T5, and T7) are normalized when divided by θz (i.e., T5/θz) where Z is dependent on the location within the engine and the engine type.
Z is usually defined by the OEM (typical values of Z range from 0.85 to 1.1).
(f) Engine cycle pressures (e.g., compressor discharge, turbine discharge, nozzle exit) are normalized when divided by δ (i.e., P3/δ).
NOTE: The subscripts “3, 4, 5, 7” are designators of specific axial stations in the engine. They are peculiar to each manufacturer. Those shown in this AC are for illustrative purposes only.
c. Fuel Properties. Experience has shown that fuel selected to a particular specification from a single supply source maintains reasonably stable properties. However, fuel taken from various sources, even when selected to the same specification, can vary in properties by several percent. When comparing fuel property data taken from different sources, the factors in the following paragraphs should be taken into consideration.
(1) Fuel Relative Density. The fuel relative density, also referred to as specific gravity, may be utilized in the fuel flow calculation as a first order correction. Therefore, accuracy of the specific gravity measurement is of prime importance when using volumetric flow measurement devices, such as turbine flowmeters. Relative density calculations are made in two steps: the relative density at a given reference temperature, then the relative density at the temperature of the fuel measured at the flowmeter. Relative density determined at the reference temperature (usually 60.0 degrees Fahrenheit [F] or 15.6 degrees Celsius [C], as provided within references of section 1-2 of Appendix 1 of this AC) is then corrected to the actual fuel temperature, using either a graph or an empirical equation. Since the relative density is a direct multiplier on the fuel flow, any error in the former will be transferred to the latter.
(2) Fuel Viscosity. Depending on the type of fuel flow measuring device utilized, the viscosity of the fuel can play a role in the calibration of that device. For example, fuel flow device calibration factors for Jet A/A1 and Jet B fuels are significantly different due to their differences in viscosity. Also, viscosity is a significant function of temperature. The specific effect can be found in the SAE ARP Report No. 4990, Turbine Flowmeter Fuel Flow Calculations.
(3) Fuel Lower Heating Value.
(a) Fuel flow measurements are corrected to a common fuel LHV base. This base is obtained by applying the direct ratio of the fuel’s LHV to the LHV of the engine model specification or to the LHV of the baseline test fuel (i.e., industry standard).
(b) Analysis of the LHV for a given batch of fuel can usually be obtained from the fuel supplier. An acceptable accuracy for LHV can be obtained using the aniline point gravity product method of a fuel sample. The determination of the aniline point and the fuel density requires a minimum of laboratory equipment. A more accurate fuel LHV can be determined in a laboratory by a precision bomb calorimeter.
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d. Ram Pressure Ratio. Defined as the engine inlet total pressure divided by the engine exhaust nozzle exit static pressure, it is the result of airflow through the test cell causing pressure differences between the front and rear of the engine. Positive ram pressure ratio values are equivalent to flying the engine at a low forward velocity that affects the engine thrust. Negative ram can be deleterious to the operation of the engine. Since the main objective of engine test cell tests is to determine static engine performance corrected to sea level, static, standard day conditions, the effects of ram pressure ratio must be measured, documented, and accounted for in thrust determination either separately or in a correlation factor.
e. Cell Depression. The difference between ambient barometric pressure and the static pressure in the test cell test chamber referenced to the engine inlet is known as cell depression. The difference is caused by pressure loss due to the velocity of air through the cell. It is usually measured in inches of water and denoted by Pcd. High levels of cell depression are usually a result of cell inlet or exhaust pressure losses. Most test cell designs result in cell depression of 2-4 inches of water, but higher or lower pressures may be acceptable. However, very low cell depression may result in vortices in the test chamber or engine exhaust gas re-ingestion into the engine inlet, causing unacceptable performance. A very high cell depression requires a large correlation factor, may indicate inadequate airflow to the engine, and can cause excessive loads on the test cell walls.
f. Cell Bypass Airflow Interactions. When a gas turbine engine is operated in an indoor test cell, its performance is altered because of the aerodynamic interference between the engine and the cell. In order to establish the thrust of the engine, the interaction of the engine with the test cell environment must be evaluated. Aerodynamic thrust corrections result from flow-induced forces within the test cell, which can be divided into three components:
(1) Inlet momentum drag;
(2) Structural drag on the engine and thrust stand; and
(3) Static pressure drag along the engine.
g. Inlet Momentum Drag. The most significant aerodynamic component of the thrust measurement is the inlet momentum, also known as the intrinsic inlet momentum. As a result of drawing air into the test cell, a force is produced on the engine. For static engine testing (i.e., non-ram facility), the magnitude of this force may be substantial; values from 1 to 10 percent of the measured thrust are typical. Since this force is, in effect, a drag term, it must be added to the measured thrust of the engine. The inlet momentum is a function of the engine inlet airflow and the approach airflow velocity in front of the engine. The approach velocity is significantly affected by the amount of cell airflow and the geometry of the test cell.
h. Structural Drag. Structural drag is generated by the cell bypass airflow scrubbing the exposed surface area of the engine casing and pushing against the exposed structure that supports the engine on the thrust measurement stand.
i. Static Pressure Drag. Local acceleration of cell bypass airflow results in static pressure gradients along projected surfaces of the engine, particularly the bellmouth and exhaust nozzle. These pressure gradients generate horizontal forces that act in a direction opposite to the measured thrust of the engine.
Static pressure drag is sometimes broken down into the bellmouth drag and boat-tail drag. The magnitude of the static pressure drag is very sensitive to the cell exhaust geometry and the spacing between the engine exhaust and the cell exhaust system.
Par 305 Page 17
j. Dress Kit Hardware.
(1) Engine performance is influenced by the airflow in the engine test section. Airflow characteristics and pressure losses in the test section are affected by protrusions in close proximity to the engine. Dress kit hardware, such as external engine cowling, bellmouth, exhaust nozzle, accessories, and test instrumentations, are examples of typical protrusions. Therefore, dress kit hardware may influence engine performance and the resulting correlation factor(s).
(2) For accuracy and consistency, the part number and serial number of each dress kit hardware item, where appropriate, and the bellmouth, fan exhaust nozzle, and primary exhaust nozzle area measurements should be documented during the correlation. Engine testing at the reference and the applicant’s facilities are generally performed using each of their respective dress kit hardware. The dress kit hardware that was used during the applicant’s correlation test should be used for all subsequent tests of the engine model in the applicant’s test cell, unless the following applies;
(a) If the applicant owns several sets of engine dress kit hardware, it may be necessary to establish correlation factors for each dress kit configuration. In this case, immediately following correlation of the applicant’s test cell, back-to-back testing using different dress kit hardware should be performed to determine if any change to the correlation factor is necessary. Test configurations need to be documented by dress kit part number, serial number, and area measurements and matched to the appropriate correlation factor.
(b) If an applicant purchases a new piece of engine dress kit hardware, a similar back-to-back test should be performed at that time to establish if the correlation factor needs to be changed.
NOTE: Paragraph 403 describes back-to-back testing.
306. DATA ACQUISITION.
a. A primary consideration of engine performance measurements should be data acquisition. When computers are not utilized in a test facility, data must be collected by visually reading and hand recording all measured parameters on a log sheet. The use of an automatic computer-operated digital data acquisition system improves the accuracy of measurements and recordkeeping.
NOTE: Technology advancement in automated data acquisition systems has given rise to potential problem areas. Close attention should be paid to potential problems in the planning stages of developing this type of system. Special attention should also be paid to cell-to-cell engine performance differences during correlation when a difference exists between data acquisition systems.
b. The following paragraphs contain items that may have a significant effect on test cell correlation.
(1) Data Scan Characteristics. Computer-operated digital data acquisition systems scan, within a short period of time, hundreds of channels of data (e.g., temperatures, pressures, vibrations, resolver angles) in short, controlled bursts of repetitive signal collection. These bursts of measured data collection may be averaged by a computer. The following additional information is offered to provide a better understanding of scan characteristics.
(a) Recommended scan characteristics may be prescribed by the OEM.
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(b) Taking multiple scans or measurements of the same parameter will obtain a better average value. Due to the total interaction of the engine control system and hardware, the performance parameters of even a well stabilized engine slowly oscillate around a steady state average. This parameter oscillation may resemble a sine wave characteristic with a period possibly lasting up to several minutes.
(c) Whether data is manually or automatically recorded, this recording process or scan takes a finite amount of time. With a rapid scan, the engine operating conditions change negligibly during a single scan period. However, with a slow scan, the sequence in which the parameters are recorded can influence the data quality due to time skew. Therefore, thought should be given to the recording sequence and parameter groupings.
(d) The period (i.e., engine cycle) of each measured parameter should be determined either analytically or empirically. This information should be used in designing a scan rate for each parameter.
Data scan techniques, including the rates, may be different for digital versus analog data acquisition systems. An ideal technique would be to record several points per engine cycle over a large number of engine cycles, so that a true average value will be calculated from the data recorded.
(e) The oscillating engine performance discussed in paragraph 306b(1)(b) is further complicated by engine thermal soak-in. Soak-in is a result of the turbine mass (particularly the disks) slowly heating up and causing reduced turbine blade-to-shroud clearances. Thermal soak-in initiates immediately upon reaching a given power point and results in a real and measurable shift in engine performance during thermal stabilization. Soak-in can take from a few minutes to as much as 15 minutes for complete thermal stabilization to occur. Data acquisition should not be initiated before full thermal stabilization. If data acquisition is initiated before full thermal stabilization, some parameters will incur a real migration in the average value (i.e., a performance shift). This can be accommodated by selectively organizing the data scan. However, as previously stated, waiting for the engine to fully soak in is more desirable.
NOTE: A selectively organized data scan should read the stable parameters that are unaffected by thermal soak-in (i.e., most pressures and cell temperature) toward the beginning of the scan after a minimal soak period, but before full thermal stabilization occurs. Soak-affected parameters (e.g., turbine temperature, core speed, fuel flow, variable vane angles) should be read toward the end of the scan.
(f) Parameters such as fuel flow, which are measured as frequency, could have significant errors in the calculated average value that are solely attributable to scan rate and sequencing. Therefore, special attention should be given to cell-to-cell engine performance differences during a correlation when a mismatch exists between the data acquisition systems. This could become particularly difficult when trying to correlate a computer-driven acquisition system against a manual or hand-logged system.
(2) Data Averaging and Editing. Once a data acquisition methodology has been selected, an averaging technique should be determined. Also, a procedure pertinent to editing the data should be considered. These parameters are then used in the performance calculations.
(a) Several averaging techniques can be employed. A straight average of all measured data is commonly performed. Alternatively, more selective techniques requiring an automatic screening of all data can be applied, with some data being classified as possibly erroneous.
(b) Averaging techniques can have a significant effect on the calculated parameter average and, thus, the resulting performance, particularly if the engine oscillates significantly or the data were acquired before full thermal stabilization. Therefore, any differences between averaging methodologies Par 306 Page 19
should be carefully considered when correlating a test cell in which different averaging techniques have been used between the test cells.
(c) Along with the averaging techniques employed, an appropriate editing procedure should be considered. Different types of editing methods can be used in conjunction with the averaging technique chosen to produce the most representative data. Editing methods include data tolerances, minimum and maximum limits, and an outlier test. Editing is especially useful on multiple pressure readings (e.g., bellmouth static or total pressures), where line leaks can affect the validity of the data.
Caution should be used when applying editing to parameters, such as fuel flow and engine exhaust gas temperatures, where editing might mask engine stability problems. When using any editing method, all measured data should be displayed and the edited data flagged so mismeasurements can be corrected.
(d) The goal of data editing is to discard the erroneous data points and use only the correct
(i.e., true) data measurements. One goal of correlation testing is to find and remedy problems in the test cell measurement systems. When data editing is used, there is risk that data showing a real mismeasurement problem may be discarded as erroneous data. If this happens, the opportunity to find and correct a measurement problem may be missed.
(3) Software Verification. Computer software may be used in data acquisition and reduction.
The FAA recommends that steps be taken to ensure that:
(a) The appropriate software program, including version (i.e., revision), is used.
(b) The software reflects the test requirements and performance calculations of the engine manual.
(c) For a given input, the reference and applicant’s facilities produce the same output.
(d) Data known to be valid can be used to verify the software functionality.
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